Sheets coated with plastics are very desirable owing to the protection from rusting which the plastics coating gives, and also due to the fact that the plastics coating enables the sheets to be given any desired colour or surface pattern. Sheets of this kind cannot however be connected with each other to form a composite sheet structure by the usual spot welding or seam welding techniques, owing to the presence of the plastics coating. I have already proposed to enable connection of such sheets, by removing the coating from the sheets at the welding regions so that when the sheets are pressed against each other, an electric contact is established.
With thick plastics coatings and also with thick sheets and other parts, the cavities which are produced by removal of the coating or by embossing or by the insertion of intermediate welding pieces, are bridged over. Consequently, with conventional welding machines, faultless welded connections can be established, even with sheets coated with plastics, lacquers or other materials, or sheets or metal parts made of steel, chrome nickel steel, anodised aluminium or other metals. However, the external coatings of the welded sheets are destroyed at the weld regions, either by having been removed to enable the welding action, or by having melted under the electrodes.
I have already proposed for the welding current to be conducted indirectly through the sheet surface, for at least one of a number of coated sheets to be welded together. As a result of this, the external coating of at least one of the sheets remains undamaged since the welding takes place internally to the opposite bare sheet surface.
When capacitor pulse welding is used, pointed formations should be provided in the cavity, on the sheets or on intermediate welding pieces disposed between the sheets.
It is disadvantageous to weaken the welding regions on the sheet the coating of which is to remain undamaged, and further difficulties are caused by the high local heating which the pointed formations result in at the weld regions.
It is an object of the invention to provide a method for the electric resistance welding of coated sheets enabling at least one of the sheets to remain undamaged at its outer side nevertheless allowing the easy provision of formations facilitating the welding.
It is another object of the invention to provide a method for the electric resistance welding of coated sheets, in which welding current is fed to at least one of the sheets indirectly through the sheet surface and electric contact is established between the sheets by ring-shaped or line-shaped contact edges formed on one of the sheets or on an intermediate piece disposed between the sheets, by embossing or milling.
With suitable formation of the contact edges thick and thin sheets or sheets of different metals can be connected together with resistance welding and with capacitor discharge welding, the heating of the sheets being so slight that the outer coatings remain undamaged. This will be explained in a series of examples.